Efficient communication of speech information often involves the coding of speech signals for transmission over a channel or network ("channel"). Speech coding systems include coding processes which convert speech signals into codewords for transmission over the channel and decoding processes which reconstruct speech from received code words. These coding and decoding processes provide data compression and expansion useful tier communication of speech signals over channels of limited bandwidth.
In analysis-by-synthesis speech coding systems, such as code-excited linear predictive (CELP) speech coding known in the art, a speech signal for coding is first divided into contiguous time segments of fixed duration referred to as subframes. Each subframe is typically 2.5 to 7.5 milliseconds (ms) in duration. Most of the speech information of each subframe is coded as a set of parameters characterizing the speech signal within the subframe. Several contiguous coded subframes (usually 4 or 6) are collected together in groups referred to as frames. These frames of coded speech are communicated via a channel to a receiver. The receiver may, e.g., synthesize audible speech from the received frame information.
A goal of most speech coding systems is to provide faithful reproduction of original speech sounds such as, e.g., voiced speech, produced when the vocal cords are tensed and vibrating quasi-periodically. In the time domain, a voiced speech signal usually appears as a succession of similar but slowly evolving waveforms referred to as pitch-cycles. A pitch-cycle waveform is generally characterized by a major transient surrounded by a succession of lower amplitude vibrations. A single one of these pitch-cycle waveforms has a duration referred to as a pitch-period.
Because of the nature of the voiced speech signal pitch-cycle, speech coding systems which operate on a subframe basis aim to accurately represent widely disparate signal features within a subframe. How these speech signal features are treated by a speech coding system significantly affects system performance.